Important characteristics of electrical energy storage devices include energy density, power density, maximum charging rate, internal leakage current, equivalent series resistance (ESR), and durability, i.e., the ability to withstand multiple charge-discharge cycles. For a number of reasons, electrochemical double layer capacitors, also known as supercapacitors and ultracapacitors, are gaining popularity in many energy storage applications. The reasons include availability of double layer capacitors with high power densities (in both charge and discharge modes), and with energy storage densities approaching those of conventional rechargeable cells.
Double layer capacitors use electrodes immersed in an electrolyte (an electrolytic solution) as their energy storage element. Typically, a porous separator immersed in and impregnated with the electrolyte ensures that the electrodes do not come in contact with each other, preventing electronic current flow directly between the electrodes. At the same time, the porous separator allows ionic currents to flow between the electrodes in both directions. As discussed below, double layers of charges are formed at the interfaces between the solid electrodes and the electrolyte. Double layer capacitors owe their descriptive name to these layers.
When electric potential is applied between a pair of electrodes of a double layer capacitor, ions that exist within the electrolyte are attracted to the surfaces of the oppositely-charged electrodes, and migrate towards the electrodes. A layer of oppositely-charged ions is thus created and maintained near each electrode surface. Electrical energy is stored in the charge separation layers between these ionic layers and the charge layers of the corresponding electrode surfaces. In fact, the charge separation layers behave essentially as electrostatic capacitors. Electrostatic energy can also be stored in the double layer capacitors through orientation and alignment of molecules of the electrolytic solution under influence of the electric field induced by the potential, but these effects are typically secondary in nature.
In comparison to conventional capacitors, double layer capacitors have high capacitance in relation to their volume and weight. There are two main reasons for these volumetric and weight efficiencies. First, the charge separation layers are very narrow. Their widths are typically on the order of nanometers. Second, the electrodes can be made from a porous material with very large effective surface area per unit volume. Because capacitance is directly proportional to the electrode area and inversely proportional to the widths of the charge separation layers, the combined effects of the large effective surface area and narrow charge separation layers result in capacitance that is very high in comparison to that of conventional capacitors of similar size and weight. High capacitance of double layer capacitors allows the capacitors to receive, store, and release large amounts of electrical energy.
As has already been mentioned, equivalent series resistance is also an important capacitor performance parameter. Time and frequency responses of a capacitor depend on the characteristic time constant of the capacitor, which is essentially a product of the capacitance and the capacitor's equivalent series resistance, or “RC product.” To put it differently, equivalent series resistance limits both charge and discharge rates of a capacitor, because the resistance restricts the current that flows into or out of the capacitor. Maximizing the charge and discharge rates is important in many applications. In electric and hybrid automotive applications, for example, a capacitor used as the energy storage device powering a vehicle's engine has to be able to provide high instantaneous power during acceleration, and to receive bursts of power produced by regenerative braking. In internal combustion vehicles, the capacitor may power a vehicle's starter, which also requires high power output in relation to the size of the capacitor.
The internal resistance also creates heat during both charge and discharge cycles. Heat causes mechanical stresses and speeds up various chemical reactions, thereby accelerating capacitor aging. Moreover, the energy converted into heat is lost, decreasing the efficiency of the capacitor. It is therefore desirable to reduce the internal equivalent series resistance of double layer capacitor cells.
Individual double layer capacitors or other energy storage cells may be combined into modules in order to raise output voltage, increase energy capacity, or to achieve both of these ends. When cells are connected in series, the resistance of the inter-cell connections effectively adds to the cells' internal equivalent series resistance. Thus, it is desirable to reduce the resistance of the inter-cell connections within a module.
Different applications may require different output voltages of capacitor modules. Similarly, some applications may need higher energy capacity than other applications. Moreover, applications may impose different constraints with respect to module volume, dimensions, and weight. Thus, it may be desirable to construct modules of different sizes and configurations, both electrical and mechanical. Customized design and production, however, are generally expensive and time consuming. Therefore, it would be desirable to reduce complexity and cost of manufacturing modules of different sizes and configurations. Furthermore, it is sometimes desirable to allow end-users to customize their modules from standardized energy storage cells.
Because energy storage modules may be moved and placed in various positions, cells within a module may need to be fastened to other cells and to the module's enclosure, so that their movement within the module is restricted or eliminated altogether. It would be desirable to do this without unduly increasing module size or weight, and without impairing effective heat conductance within modules.
Additionally, it would be desirable to increase the structural rigidity of module enclosures, and the level of physical protection provided by the enclosures to the cells disposed within the enclosures.
Conduction of heat from individual cells within modules to module enclosures, and away from module enclosures, is also an important consideration in design of modules and multi-module assemblies. It would be desirable to facilitate heat conduction from internal module cells to the enclosures and away from the enclosures. It would also be desirable to facilitate cooling of multiple modules that are placed near each other. At the same time, it would be desirable not to increase the amount of space needed for a given number of modules.